1. Field of the Invention
The present invention concerns an alarm device for computer units, which are connected in a network of twisted pair type, in which two or more conductors in the existing network are used for communication between the different computer units, where the computer units are connected via the network to a common hub so that a connection is obtained between each of the computer units via the hub.
2. Description of the Related Art
The increasing application of computers is today creating large computer networks in administrations, schools, and companies. The computers and their associated equipment must often be arranged at a number of separate locations which cannot always be kept under supervision. Since both the computers and their equipment are highly attractive to thieves, this has implied extensive theft activity concerning both computers with screens as well as components inside the computers, such as e.g. hard disks diskette drives, and memory boards.
To reduce the theft risk for computers, it has become increasingly common to provide them with some type of alarm. For example, previously known through WO-91/19277 is a fibreoptic alarm system for protection of computers. The alarm system is based on the principle that each part of the equipment is connected to an alarm center via an optical sensor and optical fibres. The document shows an alarm device per se which can be used for computer equipment but which is based on the principle that each part of the protected equipment is connected to an optical network specific to the alarm device. For this reason, this alarm device is becoming both expensive and impractical in large computer networks.
Previously known through U.S. Pat. No. 5,260,664 is an alarm device for monitoring of networks with a specific loading, such as e.g. networks of twisted pair type. The device can be used in an Ethernet network, which connects a number of units to a common hub. Changes in the network loading are detected via the alarm device according to the invention. The alarm device makes it possible to detect whether the network is intact or whether the loading in the network has decreased, which would indicate a failure somewhere in the network. This alarm is per se reliable and not especially expensive to accomplish, through a deficiency of the alarm is that information cannot be obtained about the cause of the failure in the network or where this failure has occurred. In modern extensive computer networks, it is often important to have each unit alarmed and to arrange for information about the origin of the alarm to be obtained at the alarm centre.
U.S. Pat. No. 5,231,375 is based on the principle that network-connected computer units which regularly transmit or receive signal current are connected to an alarm unit via the network. The alarm unit continuously measures the signal current which flows between two or more computer units and generates an alarm signal when this computer unit is interrupted for any time longer than that associated with normal signal failures. A deficiency is that, if the power supply to any computer unit is interrupted, the computer unit cannot transmit or receive signal current, which implies the alarm unit giving a false alarm.
Previously known through U.S. Pat. No. 4,654,640 is an alarm system used in combination with a PBX telephone system, which incorporates lines connected to protected equipments. Removal of a line is detected by a local safety monitoring device, which reports the situation to a central safety monitoring device via the PBX system. The alarm device is thus not integrated in a computer network, the invention being entirely based on access to a digital PBX telephone network.